1. Field of the Invention
The present invention relates to an active cell for a CMOS technology photosensitive sensor comprising a plurality of such cells arranged in an addressable matrix in which the cells undergo successive working cycles consisting of at least an exposure phase to detect the quantity of light that impinges on them and then a scanning phase during which the luminance information due to that quantity of light is extracted from the cell.
2. Description of the Prior Art
A prior art cell of the above type is described in a paper by Huat et al. published in the IEEE Journal of Solid State Circuits, volume 11, N° Dec. 12, 1996.
That prior art cell comprises:                a photosensitive element,        first addressable means for activating/deactivating said photosensitive element after said exposure phase,        second addressable means for extracting from the cell, during said scanning phase, the luminance signal generated by said photosensitive element,        third addressable means for transferring said luminance signal from said photosensitive element to said second addressable means;        memory means for retaining said luminance signal between the exposure phase and the time at which said cell is addressed to activate said second addressable means, and        fourth addressable means for discharging said memory means after the cell reading phase, said photosensitive element and said first, second, third and fourth addressable means being implemented in the form of semiconductor components integrated into a substrate with predetermined conductivity types.        
The above kind of cell has a shutter function that momentarily cancels the influence of the incident light during the waiting time that elapses between the exposure phase and the time at which the cell is read in the addressing order of the array in which it is incorporated. To implement this function, the luminance signal detected during the exposure phase is integrated in the memory means, which can be implemented in the form of a capacitor forming part of the cell. Because in this type of CMOS cell the luminance signal is an analog signal, it is important that the capacitor used in this way can retain the charge corresponding to the signal without distortion and without loss throughout the waiting time before the cell is read. Consequently, during this time, the capacitor must be isolated as well as possible from the remainder of the cell to prevent its charge leaking away. To this end, in the prior art cell described in the paper previously cited, the memory means implemented in the form of a capacitor are decoupled from the photosensitive element by the third addressable means immediately the exposure phase ends.
The prior art cell is implemented using transistors with the same type of conductivity (p-type conductivity), the photosensitive element being a p+ diffusion region (see FIG. 4 in the paper). The transfer means, referred to in the paper as a “shutter”, take the form of a p-type transistor whose source is a p+ diffusion region to which is connected a node at which charges integrated during the exposure phase can accumulate, for example by virtue of the inherent stray capacitance of the node. However, the source implantation of the transfer or shutter transistor can also accumulate charges and divert them toward the substrate of the cell, resulting in distortion of the luminance signal really accumulated at the node and afterwards extracted from the cell, when it is addressed to read it. Another example of a prior art cell having the drawback of integrating stray charges is described in U.S. Pat. No. 5,900,623.
To reduce the influence of stray charge accumulation in the source implantation of the shutter transistor, it would be possible to increase the capacitance of the accumulation node, but this would necessitate the implementation of a capacitive region connected to the node. This solution would have the drawback of requiring too much space on the substrate, thereby reducing the filling coefficient thereof. Furthermore, although this would reduce the influence of stray charge accumulation, it would nevertheless remain present, and would nevertheless contribute to distortion of the luminance signal finally extracted from the cell at the time it is read.